Tuning indicator tube



March 19, 1935. B. J. THOMPSON TUNING INDICATOR TUBE Filed July 25, 1954 INVENTOR BROWDER J.THQMP5ON B wfl gzu/ ATTORNEY atented Mar, 19, 1935 Radio Corporation of Delaware orto erica, a corporaion of Application July 25, 1934, Serial No. 736,833

Claims.

My invention relates to electron discharge devices, more particularly to such devices used as visual indicators of voltage and having a fluorescent electrode which is rendered luminous and a produces the appearance of a column of light the length of which depends upon the magnitude of the applied voltage, a device of this character being particularly useful as a tuning .indicator in radio receiving sets. The present invention is an improvement over a device of the general character described and claimed in a co-pending application of Rene A. Braden, Serial No. 722,746 filed April 2'], 1934 and assigned to the same assignee as the present invention.

One embodiment of such a device useful as a tuning indicator and described in the Braden application above referred to, comprises an envelope having within it a straight filamentary thermionic cathode, a fiat control grid of uniform pitch and a flat plate or anode coated with material which fluoresces under electron bombardment. In operation electrons flow from the cathode to the plate with sufiicient velocity to bombard the plate and render it luminous. The control grid is so connected in a radio receiving circuit that when the circuit is tuned to resonance with an incoming signal the luminous portion of the plate is of predetermined length. Preferably the device is so connected that the luminous portion of the anode is of minimum length when the circuit is tuned exactly to resonance.

It is an object of my invention to provide an improved electron discharge device of this general character, which is particularly suitable for use as a tuning indicator tube in a radio receiving set.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a perspective view, with parts broken away, of an electron discharge device made in accordance with my invention; Figure 2 is an enlarged partial view of the anode electrode; and Figure 3 is a schematic diagram of a portion of a radio receiving circuit using an electron discharge device such as shown in Figure 1 for indicating resonance.

50 The electron discharge device or tuning indicator tube shown in Figure l and constructed in accordance with my invention, comprises an evacuated envelope 10, provided with the usual stem and press 11 and base 12. Supported from the press is a straight cathode 13, preferably 2.

i iii (Cl. 25ll27.5)

conventional indirectly heated equipotential cathode, and a variable pitch helical control grid 1i concentric and substantially coextensive with and surrounding the cathode. The pitch of the turns of the control grid preferably varies progressively a from one end of the grid to the other, and as shown in this particular embodiment the greatest pitch is at the bottom end of the grid. Concentric and substantially coextensive with and surrounding the cathode and grid is a mesh anode 1o 15, which as best shown in Figure 2, has a coating fluorescent under electron bombardment, and to which is applied in normal operation a positive voltage with respect to the grid and cathode. Electrons emitted by the cathode and striking 15 the mesh anode cause it to become luminous wherever the electrons strike the anode.

I prefer to coat the mesh anode to make it fluorescent but fluorescence on the mesh anode may be obtained without the use of fluorescent 2o material by introducing some gas into the tube.

A visual voltage indicating tube made in accordance with my invention and having an equipotential cathode and variable pitch grid instead of a filamentary cathode and uniform pitch grid at provides a greater electron emission from the cathode which promotes increased brilliancy oi. the anode, and provides a more positive control of the electron flow from the cathode to the anode. By the use of a variable pitch grid it is possible 30 to open out or compress the scale of the luminous portion of the anode by varying the pitch of the grid. It is also possible with the variable pitch grid to provide a device which is more sensitive near the resonant point than elsewhere by using 35 suitable variations in the grid pitch.

With no negative or a small negative bias on the grid M, electrons emitted from along the entire length of the cathode 13 reach the anode 15 causing the anode to fluoresce over its entire inner 40 surface, giving the appearance of a column of light. Applying a negative bias to the grid 1t and gradually increasing the bias, decreases progressively and lengthwiseor the cathode, the electron flow from the cathode to the anode with the result 45 that the luminous portion of the anode is correspondingly decreased. This characteristic is made use of to indicate resonance in a radio receiving set.

Referring to Figure 3 where there is shown a portion of a radio receiving circuit embodying a tuning indicator tube made in accordance with my invention, the transformer 20 may be a radio frequency or intermediate-frequency transformer, the secondary of which is tuned by means of variable condenser 21. The alternating voltage which appears across the terminals of the condenser 21 and secondary of the transformer 20 when tuned to an incoming signal is applied between the plate 22 and cathode 23 of a diode detector or rectifier tube 24, the'rectified current resulting in a voltage drop across the output resistor 25, the end connected to the condenser 21 acquiring a negative potential with respect to the end connected to the cathode 23 of tube 24. The resistor 25 is by-passed for radio-frequency or intermediate-frequency currents by means of by-passing condenser 26. The voltage across resistor 25 is applied to the tuning indicator tube 10 to indicate resonance. The cathode 13 of the tuning indicator tube 10 is connected to the positive end of the resistor 25 while the grid 14 is connected to the negative end of the resistor 25. A positive voltage with respect to the voltage on the grid and cathode is applied to the anode 15 of'the tube 10.

With no voltage across the diode 24 and hence no current thru resistor 25 both ends of the re sistor 25 are at the same potential so that the grid 14 has no bias with respect to the cathode 13 of the tuning indicator tube 10. This permits the entire anode to fiuoresce since electrons reach it with sufficient velocity from along the whole length of the cathode 13 to cause it to fiuoresce. As the circuit comprising the secondary of trans former and variable condenser 21 is tuned to resonance with. an incoming signal, a voltage drop takes place across resistor 25. I As resonance is approached, the rectified current thru the diode 24 increases and the end of the resistor 25 connected to the grid 14 of the tuning indicator tube 10 becomes increasingly negative so that an increasing negative bias is applied to the control grid 14. As explained above this negative bias on the grid determines the extent of the mesh anode reached by the electron flow with sufilcient velocity from the cathode to cause fiuoresence, the bias of the grid being at a maximum negative value and the length of the luminous portion of the anode being the shortest when the detector circuit is tuned to resonance with an incoming signal.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from, the scope of my invention as set forth in the appended claims.

What I claim as new is- 1. An electron discharge device having an envelope, a cathode within said envelope for emitting electrons, a mesh anode coextensive with said cathode for receiving electrons from said cathode, a variable pitch control grid positioned between said cathode and anode for determining the extent of the mesh anode reached by the electrons emitted by the cathode, and means for causing said mesh anode to fluoresce where the electrons strike said mesh anode.

2. An electron discharge device having an envelope, a cathode within said envelope for emitting electrons, a fluorescent mesh anode coextensive with said cathode for receiving electrons emitted from said cathode and a variable pitch control grid between said cathode and said fluorescent mesh anode for determining the extent of the mesh anode reached by the electrons-emitted by the cathode.

3. An electron discharge device having an envelope, a thermionic cathode within said envelope for emitting electrons, a mesh anode concentric with and surrounding said cathode for receiving electrons from said cathode and having a coating of fluorescent material which fluoresces when electrons from said cathode strike said anode with sufiicient velocity, and a variable pitch control grid intermediate said thermionic cathode and mesh anode and concentric therewith and adapted to have a varying voltage applied thereto for decreasing progressively and lengthwise of the cathode the electron flow from the cathode to the anode, whereby the length of the fluorescent portion of said anode is determined.

4. An electron discharge device having an envelope, a thermionic cathode within said envelope for emitting electrons, a mesh anode concentric and coextensive with and surrounding said cathode for receiving electrons from said cathode, and having a coating of fluorescent material, and a control grid positioned between said cathode and said mesh anode and comprising a helical coil surrounding said cathode, the pitch between the turns of said helical coil being progressively increased from one end of the grid to the other, said control grid determining the extent of the mesh anode reached by the electron flow from the cathode when a voltage is applied thereto.

5. An electron discharge device having an envelope, a straight cathode within said envelope for emitting e1ectrons,-a fluorescent mesh anode concentric and coextensive with and surrounding said cathode for receiving electrons from said cathode to cause said mesh anode to fiuoresce, and a variable pitch grid positioned between said anode and cathode for determining the length of the portion of the anode which fluoresces.

BROWDER J. THOMPSON. 

